Devices of the aforesaid type are used for the inductive charging of a rechargeable battery incorporated into an electric vehicle. During the transmission of energy, a magnetic field with a high field strength and flux density is built up between a stationary primary coil and a secondary coil on the vehicle. This is necessary in order to induce in the secondary coil a high current sufficient for the desired transmission power.
If objects made of metal materials are introduced into the region of such a field, then eddy currents are induced in them, and those currents lead to a warming, which is dependent on the material, the introduction time, and the level of the field strength. In the presence of appropriate conditions, such an object can reach a temperature that can lead to damage, for example, melting in plastic surfaces, or to putting persons at risk. The latter appears, in particular, if the secondary side was removed and heated metal objects are freely accessible and can be touched by persons.
As a result of the characteristics of previous uses for inductive energy transmission systems, a corresponding risk by metal foreign bodies was evaluated as not relevant or, for example, with industrial vehicles (AGV), the attempt was made to remove such objects from critical field regions by brushes placed in front of the secondary consumers. With vehicles with drivers, training procedures can call attention to such objects in operation and to remove them before starting the inductive transmission or, in case of doubt, not to operate the inductive transmission. For an extensive automatic operation or with high safety requirements, of which, in particular, one can start with the use of such systems in publicly accessible areas, safety measures appear to be unsuitable or at least insufficient up to now.
From WO 2009/081115 A1, a method is known for detecting an electrically conductive foreign body on a device for the inductive transmission of electrical energy, in which, temporarily, the primary voltage is increased and the reaction of the system is observed. Normally, in this case, a corresponding reduction of the primary current takes place as a result of a secondary power regulation, which maintains the transmitted power constant. A conductive foreign body on the primary coil, however, ensures an increase of the primary power, in this case, due to a power loss, and by means of this increase, can be detected. With the detection of a foreign body, the primary voltage is turned off, so as to avoid a heating of the foreign body. In this state of the art, it is problematic that a small foreign body is difficult to detect, since the primary voltage cannot be substantially increased in comparison to its nominal value.
From WO 2005/109598 A1, a system for the inductive transmission of electrical energy to portable electronic devices, such as mobile communication devices, is known, in which, for the detection of a nonexisting secondary load or a parasitic load in the form of a foreign body, the primary power supply is briefly interrupted and the curve over time of the decay of the energy stored in the circuit is measured. Upon detecting a nonexisting secondary load or a parasitic load in the form of a foreign body with the aid of the decay process, the system goes into a standby or switch-off state, in which the power supply is still briefly turned on only during periodic intervals, so as to once more trigger a decay process and, in this way, to test whether the state that led to the standby or switch-off state continues or has changed. The standby state and the switch-off state differ merely in that in the switch-off state, a prompt calling for a removal of a parasitic load is emitted on a user interface.
A similar system is known from WO 2005/109597 A1; in that system, the decision to pass over to the standby or switch-off state is made with the aid of a comparison between the primary power delivery and the secondary power need. To this end, information regarding the secondary power need must be transmitted to the primary side. The behavior of the primary side in the standby and switch-off state corresponds to the previously mentioned state of the art, that is, a brief primary power supply takes place merely at periodic intervals, so as to determine whether the last-detected system state continues or has changed and the standby state and the switch-off state differ merely due to a display on a user interface.